1. Field of the Invention
The present invention refers to a filtering device and method for reducing noise in electrical signals, in particular acoustic (voice) signals and images.
2. Description of the Related Art
As is known, filters operating according to various linear and nonlinear techniques are used to remove undesired components from electrical signals. In particular, undesired components may be any type of noise (white noise, flicker, etc.) or other types of superimposed acoustic or visual signals.
Linear filters are at present the most widely used instruments for filtering noise. Finite-impulse filters (FIRs) eliminate all the harmonics of a signal having a frequency higher than the cutoff frequency of the filter and improve the signal-to-noise ratio (SNR). Another linear filtering technique is based on the fast Fourier transform (FFT), where the signal is transformed into the frequency domain, the undesired harmonics are removed, and then the inverse Fourier transform is calculated.
As far as nonlinear techniques are concerned, average filters are simple to design and may be implemented through simple hardware circuits. Average filters are based on the comparison of the individual signal samples in an time interval with the average of all the samples in the same time interval. On the basis of this comparison, the individual samples are selectively attenuated.
All these methods share the disadvantage that, when removing the noise, also some of the components of the original signal are removed.
Furthermore, none of the current techniques, whether linear or nonlinear ones, including average filtering, is able to preserve steep edges of the signal. If a moving-average filter is used, the width of the window must be very small if steep edges are to be preserved. However, if the size of the window becomes small, there is no significant reduction in noise energy. If linear filters are used, all the frequencies above the cutoff frequency are eliminated, with consequent marked distortion of the signal.